The present invention relates to an improved copper alloy for use in the integrated-circuit (IC) industry. More specifically, the present invention relates to an improved copper alloy which exhibits high strength as well as high conductivity so that it can be most advantageously utilized in many task specific applications such as making leadframes and other thin conducting components for high pin-number IC applications.
Copper alloys are one of the most important and ubiquitous elements in the fabrication of integrated-circuits (IC). With the rapid development of the computer and communication technology, the IC industry is experiencing an unprecedented expansion. This leads to increased demand on the quantity and quality of IC packaging technology. One of the key elements in the IC packaging technology is to develop improved leadframes to provide high quality electrical communication into and out of the semiconductor devices contained in the IC chip.
Leadframes are bridges that provide communications for electrical signals among different parts of an IC board. In addition to transporting electrical signals, leadframes also provide an important function allow efficient dissipation of heat that will be generated during the busy flow of electrons. As the trend in the IC industry is to become more highly integrated, the lines become finer, the frequency gets higher, and the cost continues to be lowered, many associated changes are also taking place. For example, plastics has taken the place of ceramics and the leadframe materials are changed from Fexe2x80x94Ni alloys to copper alloys.
A number of copper alloys have been developed for fabricating leadframes and various other applications. At the present time, there are at least sixty different commercially available copper alloys for making leadframes. Generally speaking, the copper alloys can be divided into three categories: (1) high conductivity, typically with a conductivity greater than 80% IACS, but with a tensile strength lower than 400 MPa; (2) medium-to-high conductivity and medium strength, typically with a conductivity greater than 50% IACS and a tensile strength between 400 and 500 MPa; and (3) high strength, typically for use in making leadframes for IC""s with 100 or more pins. The third type copper alloys typically have a tensile strength greater than 600 MPa but electrical conductivity of 35% IACS or higher.
The first type is commonly used in making leadframes for use in transistors. Examples of the first type copper alloys include C19210 (KFK, TAMAC4), C15100 (HCL-151, Mitsubishi C151, ZC2, etc.), C18030 (EFTEC6), etc. Examples of the second type include C19400 (Olin C194, TAMAC194, HCL194, KLF194), C18040 (EFTEC64), C19500, C19600, TAMAC5, EFTEC5, etc.; the constitute the bulk of copper alloys for making leadframes for ICs. And examples of the third type include C7025 and KLF125. Several patents discussed the C7025 alloys, these include: U.S. Pat. Nos. 4,594,221, and 4,729,372, and Taiwan Patent No. 120,435.
The following U.S. patents provide some background information on copper alloys.
U.S. Pat. No. 4,950,451 discloses and claims a copper alloy for an electronic device consisting essentially of 1.0 wt %-4.0 wt % of Ni, more than 0.2 wt % and less than 0.8 wt % of P, 0.5 wt %-6.0 wt % of Zn, 0.05 wt %-1.0 wt % of Mg, and the rest being copper and unavoidable impurties.
U.S. Pat. No. 5,064,611 discloses an claims a method for producing a copper alloy, which comprises steps of: quenching to solidify, at a cooling rate in the range from 100xc2x0 C./sec to 100,000xc2x0 C./sec, a molten metal consisting essentially of 1.0 to 8 wt % of Ni, 0.1 to 0.8 wt % of P, 0.06 to 1.0 wt % of Si, and a remainder of Cu and unavoidable impurities; and continuously cooling in succession said solidified metal to normal temperature to cause an intermetallic compound of Nixe2x80x94P and Nixe2x80x94Si to be finely and uniformed into the matrix material.
U.S. Pat. No. 5,215,711 discloses and claims an age-hardening copper alloy consisting of: (1) copper; (2) 1-2.5 wt % of Ni; (3) from more than 0.01 wt % to less than 7 wt % of Si; (4) from more than 0.01 wt % to less than 10 wt % of Fe; (5) from more than 0.01 wt % to less than 7 wt % of Ti; and (6) from more than 0.001 wt % to less than I wt % of B; wherein the amount of copper constitutes the balance of the weight of the alloy.
U.S. Pat. No. 5,248,351 discloses and claims a copper alloy for an electronic device which consists essentially of 2.0 wt %-8 wt % of Ni, 0.1 wt %-0.8 wt % of P, 0.06-I wt % of Si, and the rest being Cu and unavoidable impurties, wherein the weight of Ni, P+Si is within the range fo from 4.12: 1 to 6.06: 1; and wherein Ni5P2nd Ni2Si intermetallic compounds are present.
U.S. Pat. No. 5,250,256 discloses and claims a high-tensile copper alloy for current conduction consisting essentially of: (1) from 2.0 wt % to 4.0 wt % of Ni; (2) from 0.4 wt % to 1.0 wt % of Si; (3) 0.05 wt % to 0.3 wt % of In; (4) from 0.01 wt % to 0.2 wt % of Co; and (5) the balance of Cu.
U.S. Pat. No. 5,334,346 discloses and claims a copper alloy having high strength, enhanced ductility, and good electrical conductivity consisting essentially of (1) from about 0.5 wt % to 2.4 wt % nickel; (2) from 0.1 wt % to 0.5 wt % silicon; (3) from 0.02 wt % to 0.16 wt % phosphorus; (4) from 0.02 wt % to 0.2 wt % magnesium; and (5) the balance copper.
A number of Japanese patents also discussed copper alloys. These include JP-7-18356, JP-4-356284, JP-7-18355, JP-2522629, JP-2705875, JP-6-299275, JP-6-172895, JP-5-331574, JP-6-128708, JP-8-503022, JP-7-62504.
As discussed earlier, C7025 alloy provides medium conductivity as well as high strength. A typical C7025 alloy, as disclosed in Taiwan Pat. No. 120435, can exhibit the same kind of strength as Fe-42 Ni alloy, however, its electrical conductivity is more than 10 times better than the 42 alloy. However, its electrical conductivity can only reach 35 to 50% IACS at the maximum Because of the significance of leadframes and other electric current conducting devices in the IC industry, both commercially and technologically speaking, it is important to continue the development of other types of copper alloys which can further improve the performance and lower the cost of electronic devices.
The primary object of the present invention is to develop an improved copper alloy for use in preparing IC devices. More specifically, the primary object of the present invention is to develop an improved copper alloy with both improved strength and improved conductivity, so as to satisfy today""s need for high-conductivity and high-strength leadframes which have become a critical element in today""s IC packaging applications.
The copper alloy disclosed in the present invention consists essentially of Ni, Co, Si, (Mg and/or P), and Cu. The amount of Ni is from 0.5 to 2.5 wt %, the amounts of Co, Ni, and Si satisfy the following equation: 0.8xe2x89xa6(Ni/4+Co/6)/Sixe2x89xa61.2, the amount of (Mg and/or P) is from 0.05 to 0.15 wt %, with the balance being Cu. Furthermore, it is preferred that 2%xe2x89xa6Ni+Coxe2x89xa64%. The copper alloy of the present invention is classified, similar to C7025 alloy, as belonging to the third type copper alloy with high electrical conductivity and high strength. However, the copper alloy of the present invention exhibits unexpected superior results in that its electrical conductivity exceeds the values of 35 to 50% IACS provided by C7025, while providing the same or even better tensile strength.
In the preferred embodiment of the present invention, the copper alloy consists essentially from 0.5 to 2.5 wt % of Ni, from 0.5 to 2.5 wt % of Co, from 0.4 to 0.8 wt % of Si, from 0.05 to 0.15 wt % of (Mg and/or P), and the balance of Cu, wherein the sum of Ni and Co is between 2.0 and 4.0 wt %.
There are two alternative approaches to fabricate the copper alloy plates or strips of the present invention: a high-temperature approach and a cold-temperature.
With the high-temperature approach, copper alloys having appropriate compositions are melt using a high frequency induction furnace and then cast by rapid cooling to form ingots of desired sizes. The hot ingots are homogenized at 800 to 950xc2x0 C. for about xc2xd to 5 hours, then are immediately subjected to hot working to a rate of 70% or greater (i.e., its thickness is reduced by 70% or greater), followed by water quenching and then milled to remove oxide and scales. The alloy plates so obtained are subject to cold rolling (cold working) to a thickness reduction of 50% or greater, followed by annealing at 800 to 950xc2x0 C. for 30 seconds to 30 minutes, then they are rapidly cooled. Thereafter, the alloy plates are cold rolled again to 50% or above. The steps of annealing and cold rolling can be repeated if necessary. After the cold rolling, the alloy plates are subjected to heat aging treatment at 300 to 600xc2x0 C. for 30 minutes to 5 hours, to obtain the desired strength and current conductivity. If necessary, the aged copper alloy strips can be further subject to a small amount of cold working; however, the amount of the additional cold working should be less than 40%.
With the low-temperature approach, copper alloys having appropriate compositions are melt using a radio frequency oven and then cast by rapid cooling to form ingots of desired sizes. The hot ingots are homogenized at 800 to 950xc2x0 C. for about xc2xd to 5 hours, then are immediately subjected to hot working to a reduction ratio of at least 70% in thickness, followed by water quenching and then milled to remove oxide and scales. Thereafter, the copper plates are cold rolled a cold reduction ratio of 50% of greater in thickness, followed by aging at 300 to 600xc2x0 C. The steps of cold working and aging may be repeated so as to obtain the desired strength and current conductivity. If necessary, the aged copper alloy strips can be further subject to a small amount of cold working; however, the amount of the additional cold working should be less than 40%.
The copper alloys prepared from either process exhibits excellent electrical conductivity and tensile strength, and thus, can be advantageously used for the fabrication of leadframes for use in semiconductor industries.